Hollow core multi-mode interference optical device

ABSTRACT

A hollow core multi-mode interference (MMI) device is described that comprises a multi-mode waveguide ( 10, 14 ) optically coupled to at least two fundamental mode waveguides ( 8, 12, 16 ). The device is characterised in that it comprises a means for varying the internal cross-sectional dimensions of a portion of one or more of said at least two fundamental mode waveguides. In particular, the side wall of a fundamental mode waveguide having a substantially square cross-section can be moved using micro-electro mechanical systems (MEMS). Various optical routing devices incorporating such MMI devices are described.

This application is the US national phase of international applicationPCT/GB2003/004723 filed 3 Nov. 2003 which designated the U.S. and claimspriority of GB 0225595.8, filed 2 Nov. 2002, the entire contents of eachof which are hereby incorporated by reference.

This invention relates to a multi-mode interference (MMI) device, and inparticular to optical routing devices incorporating one or moremulti-mode interference devices.

U.S. Pat. No. 5,410,625 describes a multi-mode interference (MMI) devicefor beam splitting and recombining. The device comprises a firstcoupling waveguide and two or more second coupling waveguides that areoptically connected to a central multi-mode waveguide region. Thecoupling waveguides operate only in fundamental mode, and the physicalcharacteristics of the coupling and multi-mode waveguide regions areselected such that modal dispersion within the central multi-modewaveguide region provides for a single beam of light input in to thefirst coupling waveguide to be split into the two or more secondcoupling waveguides. The device may also be operated in reverse as abeam combiner.

Variations and improvements to the basic MMI devices of U.S. Pat. No.5,410,625 are also known. U.S. Pat. No. 5,379,354 describes howvariation of input waveguide location can be used to obtain a multi-waybeam splitter that provides division of the input radiation into outputsbeams having differing intensities. Use of MMI devices to form lasercavities has also been demonstrated; for example see U.S. Pat. No.5,675,603.

U.S. Pat. No. 5,428,698 describes various signal routing devicesincorporating hollow and solid core MMI devices. In one such device, theoutputs of an MMI beam-splitter device are optically connected via a setof relay waveguides to the inputs of an MMI beam-recombiner device. Thearrangement is such that a single laser beam input to the MMIbeam-splitter can be transferred to any one or more of the outputwaveguides of the MMI beam recombiner by the application of differentsets of phase shifts to light as it propagates through the relaywaveguides. A so-called “star coupler” device is also described in whicha single MMI device is configured to operate as both the beam splitterand beam recombiner.

U.S. Pat. No. 5,428,698 teaches how reflective or transmissive phaseshifting means may be used to introduce the required relative phaseshifts to light propagating through the relay waveguides. In hollow coredevices, the transmissive phase shifting means (typically electro-opticmodulators) are located in the hollow channels defining the relaywaveguides. The use of hollow core relay waveguides that incorporatemoveable mirrors to alter the path length of the relay waveguides,thereby operating as reflective phase shifting means, are alsodescribed.

A disadvantage of hollow core signal routing devices incorporatingtransmissive phase shifting means is that the total optical power thatcan be passed through the device is limited by the optical power thatthe transmissive phase shifting element can handle. The signal routingdevices incorporating reflective phase shifters as described in U.S.Pat. No. 5,428,698 also have certain disadvantages. For example, anoverlap of the beams propagating in the relay waveguide is required andcan introduce cross-talk that reduces the overall efficiency of thedevice. Alignment of the mirror position and accurate control of mirrormovement is also critical; any alignment errors will significantlyincrease optical losses in the relay waveguides thereby reducing theoverall efficiency of the routing device.

According to a first aspect of the present invention a hollow coremulti-mode interference device comprises a multi-mode waveguideoptically coupled to at least two fundamental mode waveguides, and ischaracterised in that the device comprises a means for varying theinternal cross-sectional dimensions of a portion of one or more of saidat least two fundamental mode waveguides.

Variation of the cross-sectional dimensions of the fundamental modewaveguides in an MMI device of the present invention provides aconvenient means of controlling the phase of light as it passes througha certain length of the fundamental mode waveguide. This phase shiftarises because the variation in the waveguide cross-sectional dimensionalters the wavelength (or phase coefficient) of the fundamental mode ofthe waveguide. The amount of variation required to impart the requiredphase shift is described in more detail below with reference to FIG. 5.

As an example, consider an MMI device operating as a beam splitter. Asingle incident beam could be split into N (where N>2) beams by modaldispersion in the multi-mode waveguide region and each of these beamscould be coupled into a fundamental mode waveguide. Variation of thecross-sectional dimensions of one or more of the fundamental modewaveguides would then permit phase differences to be introduced betweenthe N beams propagating through each fundamental mode waveguide.Similarly, in an MMI N-beam recombiner, control of the relative phase ofbeams prior to recombination in the multi-mode region would be possible.

Optical devices (routers, switches etc) fabricated using an MMI deviceof the present invention do not require the inclusion of transmissivephase shifting elements. This enables higher optical powers to behandled. Previously, moveable mirror phase shifters were implementedusing highly reflective mirrors arranged in an optical layout in whichmovement of the mirror effectively altered the path length of thefundamental mode waveguide(s). As grazing angle reflectivity isinherently higher than near normal incidence reflectivity, therequirement to provide highly reflective mirrored surfaces is overcomeby the present invention. Furthermore, the present invention does notrequire the specific optical layouts disclosed in the prior art toimplement moveable mirror phase shifting.

Variation of the internal cross sectional dimensions of the fundamentalmode waveguide may be achieved in a variety of ways. In the examplesdescribed with reference to FIGS. 1 and 3, light propagates through asubstantially square cross-section fundamental mode waveguide along thez-axis. The cross-section of the waveguide is thus in the x-y plane, andmovement of a side wall in the y-axis direction (i.e. changing the widthof the waveguide) varies the cross sectional dimensions of thewaveguide. Alternatively, or additionally, the waveguide dimensionscould be varied in the x-direction (i.e. the height of the waveguidecould be altered).

It should be noted that when hollow core optical waveguide structuresare produced, the hollow core is likely to fill with air. Herein therefractive index of the core is thus assumed to be that of air atatmospheric pressure and temperature (i.e. n≈1). However, this should beseen in no way as limiting the scope of this invention. The hollow coremay contain any fluid (for example an inert gas such as nitrogen) or bea vacuum. The term hollow core simply means a core which is absent anysolid material.

Advantageously, at least one of the fundamental mode waveguides has asubstantially square cross-section. The term substantially squarecross-section should be taken to include waveguides that areapproximately square albeit having some degree of side wall movement. Asdescribed below with reference to FIGS. 1 and 3, one or more of thehollow core substantially square fundamental mode waveguides could beformed in a base portion of material and a lid placed thereon. Thehollow waveguides would have side walls, and one or both of these couldbe moveable to allow alteration of the cross sectional dimensions (e.g.the width) of the waveguide.

Although substantially square waveguides are preferred for ease offabrication, a skilled person would appreciate that the invention couldbe implemented using fundamental mode waveguides of any shape (e.g.v-shaped, rectangular, circular, elliptical etc.).

Conveniently, the internal surfaces of the hollow core waveguides arecoated with reflective material.

As described in WO 03/065088 (the contents of which are incorporatedherein by reference thereto) a reflective coating may also be applied tothe surfaces forming the hollow core waveguides to enhance reflectivityat the wavelength of operation and thereby reduce optical lossesassociated with the device.

Conveniently, the hollow core waveguides are formed in semiconductormaterial; for example silicon or III-v semiconductor materials such asGaAs, InGaAs, AlGaAs or InSb. The semiconductor material may be providedin wafer form. Advantageously, the devices are formed usingsemiconductor micro-fabrication techniques. Preferably, suchmicro-fabrication techniques provide fundamental mode waveguides havingcross-sections of less than 3 mm, or more preferably less than 1 mm

It should be noted that the devices may be produced in a variety ofways. The waveguides may be formed in unitary pieces of material, theymay be formed from two separate pieces of material (such as a base and alid) or they may be formed from a plurality of different pieces ofmaterial (e.g. separate sections of material that, when locatedtogether, define the required fundamental mode and multi-mode waveguideregions).

Advantageously, the means for varying the cross-sectional dimensions ofa portion of said fundamental mode waveguide comprisesmicro-electro-mechanical system (MEMS) actuation means.

For example, the waveguides may comprises side walls which are moveableusing MEMS technology. The moveable portion of the waveguide may beformed as an integral part of the substrate or fabricated separately andintegrated in a hybrid fashion to form the moveable portion of thewaveguide. In the case of a device formed from a base and lid, the MEMSmeans could be formed on the lid section and integrated with the baseduring the lidding process.

Alternatively, the means for varying the internal cross-sectionaldimensions of a portion of said fundamental mode waveguide may bearranged such that the fundamental mode waveguide dimensions arevariable in response to an externally applied force. A motion ordisplacement sensor may thus be provided in which an external force isused to alter the internal cross-sectional dimensions of the fundamentalwaveguide.

According to a second aspect of the present invention a device forrouting radiation comprising at least one device according to the firstaspect of the present invention.

According to a third aspect of the present invention, an optical routercomprises at least one fundamental mode input waveguide opticallycoupled to an MMI beam splitter, the MMI beam splitter also beingoptically coupled, via two or more relay waveguides, to an MMI beamrecombiner having two or more fundamental mode output waveguides,wherein the relay waveguides comprise a means for altering the relativephases between the two or more beams propagating though the relaywaveguides such that radiation received from the fundamental mode inputwaveguide may be selectably routed to any one of the two or morefundamental mode output waveguides, and is characterised in that themeans for altering the relative phases between the two or more beamscomprises a means for varying the cross-sectional dimensions of aportion of one or more of the relay waveguides.

According to a fourth aspect of the present invention an optical routercomprises a multi-mode waveguide region optically coupled to a pluralityof input/output fundamental mode waveguides and a plurality of relaywaveguides, the router being configured to receive a beam of radiationvia one of the plurality of input/output fundamental mode waveguidesand, via modal dispersion in the multi-mode waveguide region, to dividethe received beam into a plurality of beams which are coupled in therelay waveguides, wherein the relay waveguides comprise a means foraltering the relative phases between the plurality of beams and eachrelay waveguide is terminated with a reflective means such thatradiation is returned to the multi-mode waveguide region and, dependenton the relative phases of the returned beams, routed to any one of theinput/output fundamental mode waveguides, characterised in that themeans for altering the relative phases between the plurality of beamscomprises a means for varying the cross-sectional dimensions of aportion of one or more of the relay waveguides.

According to a fifth aspect of the invention, an optical phase shiftercomprises a hollow core optical waveguide, characterised in that thephase shifter comprises means for varying the internal cross-sectionaldimensions of a portion of said hollow core optical waveguide. Theoptical phase shifter may be used in an MMI device, or any opticaldevice where a phase shifting function is required.

The invention will now be described, by way of example only, withreference to the following figures in which;

FIG. 1 shows an optical routing device according to the presentinvention,

FIG. 2 illustrates the propagation of radiation in a device of the typedescribed with reference to FIG. 1,

FIG. 3 shows a star coupler optical routing device according to thepresent invention,

FIG. 4 illustrates the propagation of radiation in a device of the typedescribed with reference to FIG. 3, and

FIG. 5 shows a theoretical plot of the amount of waveguide side-wallmovement required to impart a π phase shift to light of 1.55 μmwavelength propagating through a 10 mm long fundamental mode waveguide.

Referring to FIG. 1, an optical routing device 2 according to thepresent invention is shown. The optical routing device 2 comprises amulti-mode interference (MMI) beam splitting portion 4 and an MMI beamrecombining portion 6.

The beam splitting portion 4 comprises a fundamental mode inputwaveguide 8 that is optically linked to a multi-mode waveguide region10. The dimensions of the multi-mode waveguide region 10 are selectedsuch that modal dispersion causes the fundamental mode beam receivedfrom the input waveguide 8 to be substantially equally divided betweenthe four fundamental mode relay waveguides 12 a to 12 d (referred tocollectively as 12).

The multi-mode waveguide region 14 of the beam recombining portion 6receives light from each of the four fundamental mode relay waveguides12. The dimensions of the multi-mode waveguide region 14 are selectedsuch that, depending on the relative phase of the light received fromthe relay waveguides, a fundamental mode may be excited in any one ormore of the fundamental mode output waveguides 16 a to 16 d (referred tocollectively as 16).

A detailed description of the necessary dimensions of the multi-moderegions 10 and 14 can be found elsewhere; for example see U.S. Pat. No.5,428,698.

Control of the relative phase of light injected into the multi-modewaveguide region 14 from the relay waveguides is achieved by varying thewidth (d) of one or more of the relay waveguides by moving one of theside walls that forms the hollow waveguide. A detailed description ofthe amount of side wall movement required to impart a required phaseshift is described in more detail with reference to FIG. 5. An expandedview of the relay waveguide 12 d is also given in FIG. 1, andillustrates how the width of the relay waveguide can be varied betweend₁ and d₂.

A person skilled in the art would recognise that one side wall could bemoved as shown in FIG. 1, or that the device could be configured toprovide movement of both side walls. It would also be recognised by theskilled person that a portion of the relay waveguide could be varied inwidth whilst the width of the remaining portion is kept constant orvaried by a different amount. The phase shift introduced by side wallmovement will depend on both the amount of side wall movement and alsothe length of the portion of the side wall which is moved; this isdescribed in more detail below with reference to FIG. 5. Control of sidewall movement in the relay waveguides can thus be used to introducerelative phases shifts such that the fundamental mode beam to be coupledinto any one or more of the fundamental mode output waveguide 16 asrequired.

As described above, variation of the relay waveguide width provides aconvenient way of introducing a relative phase shift between the lightpropagating through the relay waveguides. A device of the presentinvention can thus handle higher optical power than routers of the typedescribed in U.S. Pat. No. 5,428,698 that employ transmissiveelectro-optic modulators. Furthermore, certain disadvantages associatedwith the use of moveable mirror phase shifting means (e.g. therequirement for beam cross over and the precision of mirror alignment)are also mitigated.

The relay waveguide width may be controllably varied (e.g. using an MEMSactuator) to impart a desired phase shift as described above.Alternatively, the waveguide width may be varied by an externallyapplied force and the resulting phase shift measured. A motion ordisplacement sensor may thus be provided in which an external force isused to alter the internal cross-sectional dimensions of the waveguide.

In particular, a motion and/or displacement sensor arrangement could bereadily implemented using a device of the kind described with referenceto FIG. 1 but having only two relay waveguide and two output waveguides.In such an arrangement, the phase shift imparted by changing the widthof a relay waveguide by application of an external force may be used toalter the proportion of light split between two output waveguides. Insuch a movement sensing device, a sufficiently large change in guidewidth (as described with reference to FIG. 5 below) could result incomplete switching of the output beam from one output guide to the other(i.e. a Pi phase shift). This switching effect could be measured usingphotodetectors; such photo-detectors may be integrated into the device.Multiples of the Pi phase shift could be counted as cycles of the outputbeam/signals, whilst fractions of the increment could be measured interms of the relative output powers directed to the two outputwaveguides.

Referring to FIG. 2, the basic principle underlying the multi-modeinterference effect that provides beam splitting and beam recombinationin a device of the type described with reference to FIG. 1 is shown.

FIG. 2 a illustrates transverse intensity profiles of electromagneticradiation at four positions along the multi-mode waveguide region 10 ofthe MMI beam splitting portion 4. It can be seen that the fundamentalmode input beam entering the multi-mode waveguide region 10 via theinput waveguide 8 is split into four equal intensity beams that arecoupled into the relay waveguides 12.

FIGS. 2 b and 2 c show transverse intensity profiles of electromagneticradiation at thirteen positions along the multi-mode waveguide region 14of the MMI beam recombining portion 6 with two different sets of phaseshift applied to light propagating through the relay waveguides 12.

FIG. 2 b illustrates how a first set of phase shifts applied by therelay waveguides 12 allows radiation to be routed to output waveguide 16a, whilst FIG. 2 c illustrates how a second set of phase shifts willroute radiation to output waveguide 16 b.

Radiation may also be routed to output waveguides 16 c and 16 d byapplication of appropriate phase shifts. More details concerning theappropriate phase shifts can be found in U.S. Pat. No. 5,428,698.

Referring to FIG. 3, a star coupler routing device 22 of the presentinvention is shown. The device 22 comprises input/output waveguides 24 ato 24 d (referred to collectively as 24), a multi-mode waveguide region26 and relay waveguides 28 a to 28 d (referred to collectively as 28).Each of the relay waveguides 28 a to 28 d has a moveable side wall andis terminated by fixed mirrors 30 a to 30 d (referred to collectively as30). The device 22 can thus be seen to be essentially half of the device2 with fixed mirrors added. A detailed description of the requireddimensions of the multi-mode region of such star coupler devices is alsogiven elsewhere; for example see U.S. Pat. No. 5,428,698.

In operation, radiation enters the multi-mode waveguide region 26 viaany one of the input/output waveguides 24 and modal dispersion in themulti-mode waveguide region 26 causes the radiation to be divided intofour equal intensity beams that are coupled in to each of the relaywaveguides 28. The relative phase of the fundamental mode radiationreturned to the multi-mode region by each relay waveguide afterreflection by the fixed mirrors 30 is controlled by varying the width ofthe relay waveguides. A calculation of the amount of side wall movementrequired to impart the required phase shifts is described in more detailbelow.

It should be noted that the beam passes through the relay waveguides inthe star coupler device twice (i.e. before and after reflection from themirror). Hence the induced phase shift is double that which would beproduced by a single passage through the waveguide. In a manneranalogous to that described with reference to FIG. 1, appropriateselection of the relative phase shift imparted by the relay waveguidesthus allows an output beam to be routed from the device via any one ormore of the input/output waveguides 24.

Referring to FIG. 4, transverse intensity profiles of electromagneticradiation propagating through the multi-mode waveguide region 26 of thedevice described with reference to FIG. 3 are shown.

FIG. 4 a illustrates how fundamental mode radiation coupled into themulti-mode waveguide region 26 from the input/output waveguide 24 d issplit into four equal intensity beams by modal dispersion. FIGS. 4 b and4 c illustrate how modal dispersion of the four equal intensity beams,after appropriate relative phase shifts are applied by the relaywaveguides, can form a beam in a location such that it is coupled intothe input/output waveguides 24 b or 24 a respectively. Again, it shouldbe noted that the application of appropriate phase shifts by the relaywaveguide can route an output signal to any one or more of theinput/output waveguides 24.

Although FIGS. 1 and 3 show devices providing a four waysplitting/recombining function, the invention is generally applicable toMMI devices providing an N-way splitting/recombining function where N isgreater than or equal to two. Details on the dimensions required forN-way routing are described elsewhere; for example see U.S. Pat. No.5,428,698 or U.S. Pat. No. 5,410,625.

Devices of the type described with reference to FIGS. 1 and 3 areconveniently formed as hollow channels in a silicon-based substrateusing various known semiconductor micro-fabrication techniques; forexample photolithography or deep dry etching. Silicon-based substratessuitable for use in the fabrication of such devices include bulksilicon, silicon on insulator (SOI), silicon on glass (SOG) and siliconon sapphire (SOS). Alternatively, known micro-engineering techniques(e.g. hot embossing or laser ablation) can be used to form hollowchannels in a layer, such as a polymer, located on the surface of thewafer.

The hollow channels may be provided with moveable side walls usingmicro-electromechanical systems (MEMS) technology; also termedmicrosystems technology (MST). Known actuation mechanisms used in MEMSinclude those employing electrostatic, electrothermal, electromagnetic,piezoelectric, electrostrictive, magnetostrictive, bimetallic, shapememory alloy, chemical and physical (mechanical) properties. More detailon MEMS device actuation technologies and the associated fabricationtechniques can be found in “fundamental of Microfabrication” by MarcMadou, published by CRC Press (Boca Raton) in 1997; ISBN 0-8493-9451-1.

In particular, the movable side walls may be implemented usingtechnology presently used to provide MEMS movable mirrors. For example,see “MEMS and MOEMS Technology and Applications”, P. Rai-Choudhury, SPIE(Bellingham), 2000 (ISBN 0-8194-3716-6) and “Micromachined transducerssourcebook”, G. T. A. Kovacs, McGraw Hill (New York), 1998 (ISBN0-07-290722-3). Known MEMS movable mirrors vary in size from dimensionsof the order of a few microns to several millimetres, and the amount ofmotion ranges from deep sub-micron to of the order of a few hundredmicrons. The amount of sidewall movement required to implement thepresent invention (as described with reference to FIG. 5 below) can bereadily obtained using such MEMS techniques.

It should be noted that the moveable side wall sections may be formedintegrally with the fixed side wall portions, or may be fabricatedseparately and integrated in a hybrid fashion to a substrate in whichthe fixed hollow channels are defined.

To ensure optimum device performance, the moveable side wall (orrelevant portion of the side wall) should be linearly translatablewithout inducing any significant distortion to its shape. This can beaccomplished by having an appropriate suspension system to effectivelyforce a linear motion; i.e. a system which is compliant in the directionof desired motion but stiff to other (e.g. rotational) motions. Asuitable suspension system can be provided using high aspect ratio (i.e.high depth to width ratio) springs to prevent twist. Similarly, thesuspension elements can be appropriately dimensioned with respect to themovable wall such that the translation is effectively accommodatedentirely in the suspension (e.g. much more compliant) thereby preventingdistortion.

As described in WO 03/065088 a reflective coating may also be applied tothe surfaces forming the hollow core waveguides to enhance reflectivityand thereby reduce optical losses associated with the device.

A person skilled in the art would appreciate that light can be coupledinto or out of the fundamental mode waveguides of the devices describedwith reference to FIG. 1 to 4 in many ways. For example, optical routingdevices of the present invention could be coupled to other opticalcomponents via optical fibre cables or could form an integral portion ofa photonic integrated circuit of the type described WO 03/065091, thecontents of which are incorporated herein by reference thereto.

Referring to FIG. 5, the amount of side wall movement that is requiredto impart a π phase shift to light propagating in a 10 mm longfundamental mode waveguide is shown versus the waveguide width.

It is possible to consider any change in the width of the fundamentalmode waveguide as a change in the effective refractive index of thatwaveguide. Within the paraxial approximation (i.e. where {λ/2w}²<<1.0),for light of wavelength λ, the change in phase (∂φ) of the fundamentalmode (EH₁₁) for a given change in waveguide width (∂φ) can thus beexpressed for a waveguide of length L and initial width w:

$\begin{matrix}{\frac{\partial\phi}{\partial w} = \frac{L\;{\pi\lambda}}{2\; w^{3}}} & (1)\end{matrix}$

The change in width of a fundamental mode waveguide to impart a π phaseshift to the fundamental mode radiation propagating through thatwaveguide can thus given by the expression:

$\begin{matrix}{{\partial w} = \frac{2w^{3}}{L\;\lambda}} & (2)\end{matrix}$

FIG. 5 a shows a plot of waveguide width versus the change in waveguidewidth required to provide a π phase shift to light of 1.55 μm wavelengthfor a waveguide 10 mm in length for widths up to 50 μm. For ease ofreference, FIG. 5 b provides an expanded view of the data plotted inFIG. 1 for waveguide widths up to 10 μm.

FIG. 5 thus shows that for a fundamental mode waveguide having a widthof around 10 μm, a change in width of 0.1 μm (i.e. around 1%) isrequired to impart the π phase shift required to implement an opticalrouting device of the type described with reference to FIGS. 1 and 3.This level of waveguide width variation can be readily obtained usingMEMS technology.

Although the movement of side walls of a substantially squarecross-section waveguide is described above, it should be noted that thepresent invention can be applied to waveguides of any cross-sectionalshape. It should also be noted that any dimension of the waveguide, andnot just the sidewall, may be altered to produce the required phaseshift.

1. A hollow core multi-mode interference (MMI) device comprising: atleast two fundamental mode waveguides; a multi-mode waveguide opticallycoupled to said at least two fundamental mode waveguides; and a meansfor varying the phase of light passing through at least a portion of atleast one of said at least two fundamental mode waveguides, said meansfor varying comprising a means for linearly translating both side wallsof at least a portion of one of said fundamental mode waveguides withoutsubstantial distortion.
 2. A device according to claim 1 wherein atleast one of the fundamental mode waveguides has a substantially squarecross-section.
 3. A device according to claim 1 wherein the internalsurfaces of the hollow core waveguides are coated with at least onelayer of reflective material.
 4. A device according to claim 1 whereinthe device is formed in a semiconductor material.
 5. A device accordingto claim 4 wherein the device is formed in silicon.
 6. A deviceaccording to claim 1 wherein the means for varying comprisesmicro-electro-mechanical (MEMS) actuation means.
 7. A device for routingradiation comprising at least one device according to claim
 1. 8. Adevice according to claim 1 wherein the means for varying is arrangedsuch that the fundamental mode waveguide dimensions can be varied byapplication of an external force.
 9. A hollow core optical routercomprising: an MMI beam splitter; at least one fundamental mode inputwaveguide optically coupled to said MMI beam splitter; at least tworelay waveguides; an MMI beam recombiner having two or more fundamentalmode output waveguides, said MMI beam splitter is optically coupled viasaid at least two relay waveguides to said MMI beam recombiner, whereinthe relay waveguides include a means for altering the relative phasesbetween at least two beams propagating though the relay waveguides suchthat radiation received from the fundamental mode input waveguide may beselectably routed to any one of the at least two fundamental mode outputwaveguides, wherein the means for altering the relative phases betweenthe at least two beams comprises a means for linearly translating bothside walls of at least a portion of one of said two relay waveguideswithout substantial distortion.
 10. A hollow core optical routercomprising: a multi-mode waveguide region; a plurality of input/outputfundamental mode waveguide; a plurality of relay waveguides, saidmulti-mode waveguide region optically coupled to said plurality ofinput/output fundamental mode waveguides and to said plurality of relaywaveguides, the router being configured to receive a beam of radiationvia one of the plurality of input/output fundamental mode waveguidesand, via modal dispersion in the multi-mode waveguide region, to dividethe received beam into a plurality of beams that are coupled in to therelay waveguides, wherein the relay waveguides comprise a means foraltering the relative phases between the plurality of beams and eachrelay waveguide is terminated with a reflective means such thatradiation is returned to the multimode waveguide region and, dependenton the relative phases of the returned beams, routed to any one of theinput/output fundamental mode waveguides, wherein the means for alteringthe relative phases between the plurality of beams comprises a means forlinearly translating both side walls of at least a portion of one ofsaid two relay waveguides without substantial distortion.
 11. Ahollow-core MMI beam combiner comprising: a multimode waveguide; N(N>=2) fundamental mode input waveguides optically coupled to one end ofthe multimode waveguide; and a fundamental mode output waveguideoptically coupled to the other end of the multimode waveguide thelateral positions at which said input and output waveguides are coupledto the multimode waveguide and the length the multimode waveguide beingsuch that radiation input to each of said N input waveguides may becombined by modal dispersion and intermodal interference within themultimode waveguide and coupled into said output waveguide, and whereinat least one of said input waveguides comprises means for varying thecross-sectional dimensions of a portion of that input waveguide.
 12. Ahollow-core MMI beam splitter device comprising: a multimode waveguide;a fundamental mode input waveguide optically coupled to one end of themulti-mode waveguide; and N (N>=2) fundamental mode output waveguidesoptically coupled to the other end of the multimode waveguide, thelateral positions at which said input and output waveguides are coupledto the multimode waveguide and the length the multimode waveguide beingsuch that radiation input to said input waveguide is divided into Nportions by modal dispersion and intermodal interference in themultimode waveguide, each portion being coupled into a respective outputwaveguide, and wherein at least one of said output waveguides comprisesmeans for varying the cross-sectional dimensions of a portion of thatoutput waveguide.